Original artwork is automatically copyrighted to the artist as soon as it's produced. If your artwork is original and you aren't infringing on someone else's copyright (stealing photos to use in your work, etc), then you legally already own the copyright.
If you want to make it more official with documentation you could file a copyright on your work at the U.S. Copyright Office website, but you'll have to pay a fee. Possibly worth it if you are a professional artist working commercially, but in general it's like verifying ownership that you already technically have The only difference is that there is an official public record that could speed things up if there was a lawsuit filed.
Otherwise like Ryan said you can water mark it and upload a lower quality that can't be reproduced commercially and still look good. That's about it. People will always find ways to steal work. I think you just have to kind of live with that as an artist uploading stuff to the internet.
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